The present invention relates generally to antennae loaded by one or more meanderlines (also referred to as variable impedance transmission lines or slow wave transmission lines), and specifically to such an antenna providing multi-band and wide band operation and presenting a low profile.
It is generally known that antenna performance is dependent upon the antenna shape, the relationship between the antenna physical parameters (e.g., length for a linear antenna and diameter for a loop antenna) and the wavelength of the signal received or transmitted by the antenna. These relationships determine several antenna parameters, including input impedance, gain, directivity and the radiation pattern shape. Generally, the minimum physical antenna dimension must be on the order of a quarter wavelength of the operating frequency, which advantageously limits the energy dissipated in resistive losses and maximizes the energy transmitted. Quarter wave length and half wave length antenna are the most commonly used.
The burgeoning growth of wireless communications devices and systems has created a significant need for physically smaller, less obtrusive, and more efficient antennae that are capable of operation in multiple frequency bands and/or in multiple modes (i.e., different radiation patterns). Smaller packages do not provide sufficient space for the conventional quarter and half wave length antennae. As is known to those skilled in the art, there is an inverse relationship between physical antenna size and antenna gain, at least with respect to a single-element antenna. Increased gain requires a physically larger antenna, while users continue to demand physically smaller antennae. As a further constraint, to simplify the system design and strive for minimum cost, equipment designers and system operators prefer to utilize antennae capable of efficient multi-frequency and/or wide bandwidth operation. Finally, it is known that the relationship between the antenna frequency and the antenna length (in wavelengths) determines the antenna gain. That is, the antenna gain is constant for all quarter wavelength antennae of a specific geometry (i.e., at that operating frequency where the effective antenna length is a quarter of a wavelength).
One prior art technique that addresses some of these antenna requirements is the so-called xe2x80x9cYagi-Udaxe2x80x9d antenna, which has been successfully used for many years in applications such as the reception of television signals and point-to-point communications. The Yagi-Uda antenna can be designed with high gain (which is directly related to the antenna directivity) and a low voltage-standing-wave ratio (i.e., low losses) throughout a narrow band of contiguous frequencies. It is also possible to operate the Yagi-Uda antenna in more than one frequency band, provided that each band is relatively narrow and that the mean frequency of any one band is not a multiple of the mean frequency of another band. That is, a Yagi-Uda antenna for operation at multiple frequencies can be constructed so long as the operational frequencies are not harmonically related.
Specifically, the Yagi-Uda antenna includes a single element driven from a source of electromagnetic radio frequency (RF) radiation. That driven element is typically a half-wave dipole. In addition to the half-wave dipole element, the antenna includes a plurality of parasitic elements, including a reflector element on one side of the dipole and a plurality of director elements on the other side of the dipole. The director elements are usually disposed in a spaced-apart relationship in the direction of transmission (or in the direction from which the desired signal is received when operating in the receive mode). The reflector element is disposed on the side of the dipole opposite from the array of director elements. Certain improvements in the Yagi-Uda antenna are set forth in U.S. Pat. No. 2,688,083 (disclosing a Yagi-Uda antenna configuration to achieve coverage of two relatively narrow non-contiguous frequency bands), and U.S. Pat. No. 5,061,944 (disclosing the use of a full or partial cylinder partially enveloping the dipole element).
U.S. Pat. No. 6,025,811 discloses an invention directed to a dipole array antenna having two dipole radiating elements. The first element is a driven dipole of a predetermined length and the second element is an unfed dipole of a different length, but closely spaced from the driven dipole and excited by near-field coupling. This antenna provides improved performance characteristics at higher microwave frequencies.
One basic antenna model commonly used in many applications today is the half-wave dipole antenna. The radiation pattern is the familiar donut shape with most of the energy radiated uniformly in the azimuth direction and little radiation in the elevation direction. The personal communications (PCS) band of frequencies extends from 1710 to 1990 MHz and 2110 to 2200 MHz. A half-wavelength dipole antenna is approximately 3.11 inches long at 1900 MHz, 3.45 inches long at 1710 MHz 2.68 inches long at 2200 MHz, and has a typical gain of a 2.15 dBi. A derivative of the half-wavelength dipole is the quarter-wavelength monopole antenna located above a ground plane. The physical antenna length is a quarter-wavelength, but the ground plane influences the antenna characteristics to resemble a half-wavelength dipole. Thus, the radiation pattern for such a monopole above a ground plane is similar to the half-wavelength dipole pattern, with a typical gain of approximately 2 dBi.
The common free space (i.e., not above ground plane) loop antenna (with a diameter of approximately one-third the wavelength) also displays the familiar donut radiation pattern along the radial axis with a gain of approximately 3.1 dBi. At 1900 MHz, this antenna has a diameter of about 2 inches. The typical loop antenna input impedance is 50 ohms, providing good matching characteristics. Another conventional antenna is the patch, which provides directional hemispherical coverage with a gain of approximately 3 dBi. Although small compared to a quarter or half wave length antenna, the patch antenna has a low radiation efficiency.
The present invention is an antenna comprising a ground plane, one or more conductive elements, including a horizontal element and at least two spaced apart vertical elements each connected to the horizontal element by a meanderline coupler. The meanderline coupler has an effective electrical length through the dielectric medium that influences the overall effective electrical length, operating characteristics and pattern shape of the antenna. Further, the use of multiple vertical elements or the use of multiple meanderline couplers on a single vertical element provides controllable operation in multiple frequency bands. An antenna comprising meanderline couplers has a smaller physical size, yet exhibits enhanced performance over a conventional dipole. Further, the operational bandwidth is greater than typically encountered with a patch antenna. Finally, an antenna constructed with two properly-oriented horizontal elements and therefore four meanderline couplers (two for each horizontal element) in accordance with the teachings of the present invention offers polarization diversity, including providing a circularly polarized signal. Polarization diversity depends on the phase relationship between the signals input to the two antennae and the physical orientation of the radiating elements. According to the antenna reciprocity theorem, the antenna exhibits the same polarization characteristics in the receiving mode as it does in the transmitting mode. For example, circular polarization is achieved by coupling two meanderline antennae together wherein the meanderline antennae are oriented 90 degrees orthogonally to each other and further wherein the transmitted or received signal is combined using a hybrid phase combiner. A single meanderline antenna provides linear polarization of the transmitted signal and receives linear polarized signals.
In one embodiment, a meanderline coupled antenna operates in two frequency bands, with a unique antenna pattern for each band (i.e., in one band the antenna has a omnidirectional donut radiation pattern (referred to herein as the monopole mode) and in the other band the majority of the radiation is emitted in a hemispherical pattern (referred to as the loop mode). According to the teachings of the present invention, the antenna comprises horizontally stacked meanderline couplers providing a meanderline-loaded antenna having a lower profile (i.e., a smaller vertical height) than the prior art meanderline-loaded antennae. The incorporation of antennae into mobile and hand-held devices requires an antenna having a low profile configuration so that the antenna occupies less space than antennae constructed according to the teachings of the prior art.